In the prior art various examples of Chromatography columns are known to be designed for direct heating by means of conductive elements to which electrical power is applied in a controlled way in the attempt to obtain the desired temperatures.
U.S. Pat. No. 5,611,846 by Overton et al. describes a chromatography column assembly with direct heating. This document suggests inserting the capillary column and a conductive filament into an insulating sheath. In practice this solution is subject to phenomena of dispersion and has limits both as regards the maximum length of the column (2-3 meters at the most) and as regards the maximum temperature that can be reached (250° C. at the most).
Another example is constituted by the column assembly described in U.S. Pat. No. 5,808,178 in the name of Thermedics. Among the various embodiments illustrated, it suggests inserting a capillary column made of fused silica into a steel tube. The tube is then coated with an insulating sheath made of woven glass fibers. However this embodiment requires high electrical powers (up to 2 kW) to be applied to the steel tube in order to heat the column. This also makes the position of the column inside the tube difficult to reproduce and makes the temperatures dependent on the distance between the column and the wall of the steel tube.
U.S. Pat. No. 6,217,829 by Mustacich et al. describes a column assembly with heating wires and sensors which to overcome the limits of the Overton solution, are wound tightly in a spiral forming a kind of toroidal shaped pack insulated towards the outside. Due to the casual arrangement of the heating wires in the packing, excessive heatings may occur in specific points of the column, just as in other points the temperature may be lower than that desired. In the same way, temperature detection along the entire column is influenced by the casual distribution of the conductive filaments foreseen for this purpose. This technology is complex and costly to produce. Moreover, the non-uniformity of thermal dispersions between the “core” and the outer surface of the assembly may cause non-uniformity of the temperature in the section of the toroid. Another example of a known type of column assembly is described in the international patent application n. WO 02/40988 in the name of the Applicant.
The column assembly described in this prior art document comprises a capillary column and a single element in an electrically conductive material placed in contact with it. The electrically conductive element is constituted in particular by a plurality of filaments in electrically conductive materials that are woven together to form a tubular mesh that envelops the capillary column. The conductive element produced in this way, which may have filaments in different electrically conductive materials, is used both to heat the column, supplying electrical power to its ends, and to detect the instantaneous temperature of the column by measuring an electrical quantity at its ends.
The assembly constituted by the column and by the electrically conductive tubular mesh is then covered with an outermost tubular mesh, produced by weaving together filaments in electrically insulating material.
From the theoretical point of view, this solution offers considerable advantages compared to the direct heating systems mentioned above, but may not be very versatile from a practical point of view. In fact, it is often necessary to obtain columns of pre-established length with conductive elements having specific electrical properties, for example overall resistance, specific linear resistance and the like. For example, the resistance range of the conductive elements of the column must be established according to the control system. This means that it is necessary to appropriately choose the conductive material to be used, or the diameter of the filaments to be woven to obtain the desired conductive element associated with the column of pre-established length.
Nonetheless, the need to obtain these specific electrical characteristics or properties may also require the use of filaments produced with conductive materials that are not easily workable, or the use of conductive filaments whose diameter is not particularly suitable to be woven or not particularly suitable to provide the finished product with specific dimensions and/or mechanical resistance.